What’s Vulnerable › Vulnerabilities

CVE-2025-47278: Flask uses fallback key instead of current signing key

• Severity: low (CVSS 1.8 v4.0)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.2% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2025-05-13 · updated 2026-06-17
• Weakness: CWE-683

Description

Flask is a web server gateway interface (WSGI) web application framework. In Flask 3.1.0, the way fallback key configuration was handled resulted in the last fallback key being used for signing, rather than the current signing key. Signing is provided by the `itsdangerous` library. A list of keys can be passed, and it expects the last (top) key in the list to be the most recent key, and uses that for signing. Flask was incorrectly constructing that list in reverse, passing the signing key first. Sites that have opted-in to use key rotation by setting `SECRET_KEY_FALLBACKS` care likely to unexpectedly be signing their sessions with stale keys, and their transition to fresher keys will be impeded. Sessions are still signed, so this would not cause any sort of data integrity loss. Version 3.1.1 contains a patch for the issue.

Affected products

• pallets flask: = 3.1.0

References

• https://github.com/pallets/flask/commit/73d6504063bfa00666a92b07a28aaf906c532f09
• https://github.com/pallets/flask/releases/tag/3.1.1
• https://github.com/pallets/flask/security/advisories/GHSA-4grg-w6v8-c28g

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